1. A relatively uniform flat surface surface which is warmed by the sun
2. A large change of temperature with height (that promotes vertical instability and rising thermals)
3. Sunny or partly cloudy skies
4. Light winds in the region, since strong winds tend to interrupt or distort thermal formation.

The whirlwind making landfall

The location and time in question had a lot going for it, if you like a dust devil.

There was an extended flat, uniform area (in Marymoor Park) just to the northwest of the landfall site.... I suspect that is where the whirlwind was generated. Here is shot from the northern portion of the flat area. Dust devil territory for sure.

The regional winds were very light in the area...with local observations around Lake Sammamish showing calm conditions (see surface weather map at 5 PM Monday)

The day started out nearly clear and became partly cloudy as the air became less stable--so there was sun to warm the surface.

And there was a large change in temperature with height (known as a large lapse rate). Here is a time-height cross section at Seattle Tacoma Airport, showing temperatures (red lines) and winds with height for the 24 h period ending 11 PM Monday. Between temperatures cooling aloft, while they warmed at the surface, a large decrease of temperature with height was apparent.

Looking at the observations at Marymoor Park, temperatures hit the maximum (around 70F), just before the whirlwind formed.

So all the ingredients for a dust-devil were in place, with the area downwind of the flat southern portion of Marymoor Park being a favorable location, a large change in temperature with height, light winds in the region, and enough sun to warm the surface.

This whirlwind was scary for both man and beast, terrifying some poor dogs on the dock.

And the raft that was lofted high in the sky? After traveling hundreds of feet and about 50 ft into the air, it came down on a lot several houses down the street. I bet that mystified the home owner.